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Estuaries – Conduits between Land and Sea

By Paul Caiger.

What is an estuary? An estuary is many things: a home, a nursery, a filter, a buffer, a destination. Estuaries are a place of transition, where the land meets the sea, and freshwater meets saltwater. Above all else, estuaries are dynamic.

Derived from the term aestus, meaning tide, the daily flux of the tides are forever shifting the channels and sandbanks. Rivers bring in sediments; the ocean brings in shell and sand; the tides move them around. This mixing of water bodies fosters a highly productive environment, some 20 times more productive than the open ocean. Consequently, they are places rich in plant and animal communities, and fantastic places to explore.

In New Zealand there are around 300 estuaries, coming under many other monikers: bays, harbours, inlets, lagoons, to name but a few. They vary in shapes and sizes, ranging from small tidal inlets, to the largest harbour in the southern hemisphere, the Kaipara Harbour. Most of the New Zealand’s estuaries formed when the last ice age receded somewhere around ten thousand years ago, flooding the glacial-scoured river valleys and bays. Others have formed over more recent times due to the shifting of bars or other harbour-creating barriers.

It is the physical nature of estuaries that make them so unique. With maritime and riverine inputs, a unique salt wedge is formed, where the lighter freshwater sits atop the heavier saltwater. This wedge moves up and down the estuary with the fluxes of the tide, constantly altering the brackish composition. The constant salinity fluxes, in addition to other fluctuating conditions such as water level, oxygen content and temperature, all contribute to this being an unforgiving place to live. It is no surprise then, that they are inhabited by resilient organisms.

When the tides pour out, and the mud and sand flats are left high and dry, myriad plants and animals are left to deal with the perils of exposure, both to predation and desiccation. Many burrow themselves. Others avoid this dramatic shift in salinity by regulating their internal composition, termed osmoregulation. When the tide returns, the hidden creatures resume life at a frantic pace, filtering the phytoplankton-rich water, amongst other daily routines. Next to phytoplankton, the two most important plant species are seagrass and mangroves, providing vital habitat for organisms. Equally important, they also sequester and store large quantities of carbon from the atmosphere, what is known as ‘blue’ carbon.

The world’s only estuarine triplefin, Forsterygion nigripenne.

The world’s only estuarine triplefin, Forsterygion nigripenne.

Some species use the unique conditions as an opportunity to exploit a new niche, branching off from close relatives. Such an example is the estuarine triplefin (Forsterygion nigripenne), the only triplefin in the world that has adapted to life in the brackish estuarine waters. Likewise, when uninvited visitors arrive by human means, and they are suitable to the conditions, then they can also exploit them, and abound unchecked. The bridled goby (Arenigobius bifrenatus) and oyster blenny (Omobranchus anolius) are two such fishes that have been found spreading through our northern estuaries. Though their effects are as yet unknown, sizeable predatory fishes such as the bridled goby are sure to have some impact. A common means of introduction are as hitchhikers on ships and in ballast waters.

The advent of the Auckland harbour bridge extensions in the late 1960s brought with it the Pacific Oyster (Crassostrea gigas). This oyster has misplaced many native species, and even reshaped many Waitemata Harbour reefs, turning communities once dominated by native tubeworms and bivalves into oyster smothered reefs.

Not only does the ever-changing environment lead to novel adaptations for survival, it also leads to very different experiences for the avid explorer. Walk out onto the tidal flats at low tide, and it can seem barren, save the mantis shrimp and crab holes and gastropods bustling around. The scalloped shape of a ray pit might reveal what had been feeding there hours previous. But, when the tide returns, estuaries are one of the best places to snorkel, and even dive. The estuary is alive and teeming with life; schools of planktivorous fishes swarming the surface, rays and flatfish extending to the far reaches to hunt for crustaceans, all in a rush before the turn of the tide. Looking closer, an abundance of macro life can be found camouflaged against the sand or peering out from what little structure there may be, from sand-coloured gobies to any manner of invertebrates.

Estuaries are also under increasing pressure from anthropogenic stressors, and we are currently in a crossroads with their health at the junction. A focal point for human populations due to sheltered waters and access to resources, the large majority of Kiwis live on or near an estuary or harbour – as a result, many suffer degradation.

Overharvesting is the obvious side effect of dense populations living nearby, but the two primary concerns are from sedimentation and nutrients. Sedimentation occurs from soil erosion, deforestation and land development. The infilling of estuaries has accelerated hugely since the arrival of people, with an estimated build-up of sediment from less than a millimetre a year, to more than 20mm in some places now. The Waitemata Harbour has accumulated metres of sediment, and many areas will soon rise above the high-tide level.

Yellow-eyed mullet (Aldrichetta forsteri) feeding on plankton.

Yellow-eyed mullet (Aldrichetta forsteri) feeding on plankton.

In addition, nutrients from limitless activities end up in estuaries, resulting in eutrophication. Urban areas gift paints, fuels and sewage; rural areas, pesticides and fertilisers. Catchment areas can be enormous, and nutrients can get funnelled to the ocean from many kilometres from an obvious estuary or river. Bad farming practices also hurt our galaxiids, known to most Kiwis as whitebait fritters. By not fencing or planting the margins of estuaries, or the streams that lead to them, crucial galaxiid spawning habitat is damaged by stock, meaning fewer kokopu in our waterways, and fritters on our plates.

In the past, many Kiwis failed to appreciate the value of estuaries, as their life in the towns and on farms seemed removed from the state of coastal waters. But with an expanding population demanding clean water for recreation, harvest and aquaculture, people are learning that healthy estuaries are an asset and deserve careful management. Take time to explore an estuary close by and enjoy the unique flora and fauna that abide. Making these fascinating places a destination can only increase their value, and help safeguard them for future generations to enjoy.

Correction:

In our last issue 152, Feb/March, we ran a story by Paul Caiger entitled ‘Pelagic Magic’ where we incorrectly identified the coral-banded shrimp as a pelagic animal. We apologise for any confusion this has caused – the mistake is ours rather than the author’s.

 

 

 

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